A variety of centrifugal pumps are known which are capable of pumping liquids and fluid containing solid matter such as small pieces of garbage or other disposed items. The known chopper pumps have the capability of chopping or cutting solid matter in the liquid mixture, permitting the output from the pump to be disposed of more readily. One known chopper pump is sold by Vaughan, Inc. This pump is provided with a so-called “flushless” mechanical seal which is a cartridge type seal with a coil spring for the seal extending around an inner sleeve that is mounted on the drive shaft. This cartridge seal is mounted in an enclosed chamber formed in a back plate of the pump and extending rearwardly therefrom. The impeller mounted on the drive shaft is a shrouded impeller and the impeller and pump casing are constructed in a manner to keep solids and debris out of the enclosed chamber.
Another known centrifugal pump construction sold by Cornell Pump Company of Portland, Oreg. employs a shrouded impeller have backvanes formed thereon and an enclosed chamber formed behind the shrouded impeller which contains a mechanical seal and an exposed coil spring that engages one side of the mechanical seal and the back of the impeller. The seal chamber is relatively open at the front end but is covered by the impeller. The chamber converges in an axial rearwards direction and has a circumferential wall formed with deflector vanes. According to the manufacturer of this pump, the deflector vanes work with the impeller backvanes to create a fluid action behind the impeller which removes solids and abrasive material from the seal area. However, the impeller used in this pump is a shrouded impeller and the back shroud substantially restricts the flow of fluids into the seal chamber.
Although the prior art pumps having seal chambers with mechanical seals and shrouded (closed) type impellers have been provided in some cases with features intended to keep solids and debris away from the seal chamber, the problem of keeping solids and debris away from the seal chamber is very challenging in the case of pumps employing shrouded impellers because a significant pressure gradient exists between the periphery of the impeller shroud (high pressure) and the seal chamber (low pressure). It will be appreciated that with these pump designs, the natural inclination of the fluid and debris is to go from the high pressure area within the pump towards the low pressure area of the seal chamber. Furthermore, once solids enter the seal chamber, it is difficult to move them out against the high pressure.
In the case of known centrifugal pumps employing a so-called “flushless” seal design together with a shrouded impeller, the makers of these pumps endeavor to prevent solids from getting to the seal chamber. This particular problem has presented difficulties for these pump manufacturers and the cooling of the mechanical seal can often be compromised because fluid circulation through or around the seal is restricted.
The centrifugal pump disclosed herein takes a different approach than prior art centrifugal pumps by providing a central seal chamber which forms an open annular space behind the impeller for circulation of a portion of the fluid in the region of the mechanical seal and by allowing fluid to circulate in the space by using an open impeller construction. In the presently disclosed chopper pump, the chamber and the mechanical seal mounted therein are exposed to the pumped media to a considerable extent and because of the enhanced flow around the mechanical seal, the seal can be effectively cooled and lubricated.